1. Field of the Invention
The present invention relates to a method for producing graft-modified copolymers which are obtainable by subjecting an unsaturated copolymer comprising an .alpha.-olefin and a specific cyclic olefin and a radically polymerizable monomer to the radical polymerization conditions, and which are excellent in adhesion, printability, hydrophilicity, and compatibility in a polymer blend.
2. Related Art
Homopolymers of an .alpha.-olefin, which herein includes ethylene, and copolymers thereof are not only inexpensive but also excellent in mechanical strength, gloss, transparency, moldability, resistance to humidity, and resistance to chemicals, so that they are widely used either singly or as a component of a polymer blend. However, since the .alpha.-olefinic polymers have a non-polar molecular structure, they are poor in affinity for other materials and extremely inferior in adhesion, printability, and compatibility in a polymer blend.
In order to improve the above properties, a method in which a modified polymer is prepared by graft-polymerizing a radically polymerizable monomer to an .alpha.-olefinic polymer has long been attempted repeatedly.
However, such a radical grafting method still has a lot of problems. For example, this method cannot give a sufficiently high graft ratio or graft efficiency. Further, in most cases, a radical polymerization initiator such as an organic or inorganic peroxide is used in this method, so that a backbone polymer to which a monomer is grafted readily undergoes a molecular cleavage or a crosslinking reaction.
In order to solve the above problems, a variety of inventions have been proposed. Of these, those inventions which are disclosed in Japanese Laid-Open Patent Publications Nos. 98508/1982 and 269110/1990 are considered to be particularly related to the present invention. In these inventions, the graft ratio of a monomer is increased by restraining a backbone polymer from a molecular cleavage or crosslinking by making use of the reactivity of unsaturated bonds contained in an unsaturated copolymeric resin composed of an .alpha.-olefin and an unconjugated diene monomer.
However, the unsaturated copolymers for use in these inventions are prepared by using an unconjugated diene monomer which is not always highly copolymerizable. Further, from the viewpoints of the graft ratio of a monomer, the graft efficiency, a molecular cleavage of a backbone polymer, formation of gel in a radical graft reaction product and the like, it cannot be said that the radical graft reaction in which a monomer is grafted to such an unsaturated copolymeric resin has been practically perfected.
On the other hand, Japanese Patent Publication No. 19854/1971, and Japanese Patent Laid-Open Publication Nos. 241907/1987 and 87610/1989 disclose copolymers of an .alpha.-olefin, particularly propylene, with divinylbenzene. It is, however, afraid that the copolymers may contain a gel fraction formed during copolymerization and that the copolymers may not produce, upon modification, graft-derivatives endowed with desirable properties.
The foregoing prior art is useful in its own way. However, it is unsatisfactory in the graft efficiency and the like.